The dimension measurements, e.g. evaluations of critical dimension (CD) and d spacings, in TEM cannot be accurate (normally 10% ~ 100% inaccuracy) if the camera length is in error. To ensure that the installation-calibrated camera length can be used in your experimental measurement, the specimen should be positioned in the "Eucentric plane". This can be done simply by manually tilting the specimen (or by clicking the proper button on user interface, e.g. “Wobbler” pointed by the red arrow in the FEI interface in Figure 2680) and thus finding the height of the specimen holder where the image of the specimen remains stationary.
Figure 2680. “Wobbler” button for Eucentric height adjustment on FEI user interface.
On the other hand, the Eucentric position also means that the specimen height is adjusted to be on the tilt axis of the goniometer stage. This can be done, for instance, in FEI TEMs by using the alpha wobbler, L2, to tilt the sample back and forth and adjusting the “Z axis” up or down to minimize image movement.
Note that for TEMs with top-entry stages, it is usually noneucentric, i.e. the specimen translates out of the selected area if it is tilted.